The large size of native format Affinity files is a frequent topic in these forums. See for example the comments in this one for an explanation of why they are so large.
The large file sizes happen because our file format is optimised for loading speed rather than size. The original images were probably JPEG, but in we store them as ZIP which does not really compress photographs at all. To add insult to injury, we typically store the JPEG as well, and then store multiple copies of the image at successively smaller resolutions.
There was a plan to have a "minimise space" option on the Save dialog, but it's not happened yet. I think we will have to do something before Publisher, because Publisher documents could be hundreds of pages and have a lot of images.
